Electrical-motor-driven devices have found use in a wide variety of applications. Many of these applications require regular monitoring to ensure that the motor is properly functioning. Motor current spectrum analysis (MCSA) has been developed as an effective method for efficiently, reliably, and non-intrusively monitoring the condition of electrical-motor-driven devices. MCSA permits individuals to determine the operating condition of rotating equipment. The operating condition can then be related to the maintenance needs of the equipment.
MCSA is based upon the transduction of signals back to the power line via an electrical motor. For example, mechanical vibrations in alternating current (AC) rotating equipment are transduced back to the power line via the electrical motor. Additionally, electrical characteristics of the motor are transduced back to the power lines. These transduced signals are very small modulators to the large AC power line current.
The signals necessary for MCSA are typically provided by attaching a current transformer to a lead of the electrical motor. Once the signals are gathered, they are conditioned, sampled and analyzed in the frequency-domain with the Discrete Fourier Transform (DFT). Any periodic time-domain vibrations and fault data produced by the motor are displayed as peaks in the frequency spectra. However, the larger signal produced by the large AC power line current, and its harmonics, are also displayed in the frequency spectra at a magnitude that can be several orders of magnitude greater than the signals of interest. The spectra of motor current data for these large power line frequencies are expansive and Gaussian in nature when they are sampled by conventional methods. As a result, any anomalies or abnormalities having a frequency close to the frequency of the AC power line current are difficult to evaluate and precisely define.
All induction type electrical motors include two common features. First, each includes a stator which is a stationary element through which externally supplied current is passed. In addition, each motor includes a rotor which is a rotating element into which an electric current is induced. Common commercial motors create a magnetic field by passing electrical current around multiple stator slots. The magnetic field induces currents within rotor bars, contained in the rotor, which provide the electrical conduction path for the induced current.
As an individual rotor bar enters a stator pole's field of influence, the magnetic coupling of the stator and rotor changes. The resultant change in impedance seen by the stator produces a small change in the current flowing from the power supply to the stator. As a result, small high frequency signals are produced by the rotation of the rotor bars. These small high frequency signals modulate the large AC power line current.
Broken rotor bars are a common form of degradation/failure in electric motors. Rotor bars can fail from a variety of causes, including thermal, mechanical and chemical breakdown. In addition, manufacturing defects can result in imbalances in load sharing among the individual rotor bars.
Existing methods and apparatuses for monitoring rotor bars have relied solely on secondary indicators. For example, slip frequency magnitude has been used to detect rotor bar degradation. Slip, which is a measure of the difference between the rotating speed of the motor and its synchronous speed, has been measured by either magnetic fields in the vicinity of the motor or by overall current measurements (either in the raw or demodulated forms).
For example, U.S. Pat. No. 4,965,513, to Haynes et al., and U.S. Pat. No. 4,978,909, to Hendrix et al., disclose the use of spectral analysis to monitor electrical motors and the associated driven devices. Both methods focus on the low frequency content of the motor current, either by demodulating the overall current signal (Hendrix et al.) or by collecting synchronous data (i.e., synchronous to the AC electrical supply current which is a 60 Hz carrier with regard to the resultant signal produced by the motor current; see Haynes et al.). In the case of the synchronous data collection system, the existing practice is to sample the unfiltered data at a moderate frequency of 1920 Hz, thus limiting the ability of the system to observe high-frequency related phenomena.
Additionally, the existing techniques, such as measuring the energy content of slip sidebands of 60 Hz or slip magnitude in the demodulated motor current, are inherently limited by the dynamic range of the recording media. Further, the raw motor current slip magnitude has been shown to not always be a faithful indicator of rotor degradation.
U.S. Pat. Nos. 5,030,917 and 5,049,815, to Kliman disclose methods for monitoring rotor faults in induction motors by considering the current drawn by a motor. Specifically, Kliman discloses measuring characteristics of the current drawn when an electrical motor is started to determine rotor faults. For example, in U.S. Pat. No. 5,030,917 the drawn current is analyzed to determine dips in the starting motor current amplitude when the motor reaches approximately half speed. In contrast, U.S. Pat. No. 5,049,815 discloses the conversion of signals indicative of the drawn current to generate a frequency spectrum of the motor current. The frequency spectrum is then analyzed to determine rotor fault and slip frequency values. Briefly, the signals from the drawn current are fed through a low pass/amplifier to remove unnecessary high frequency components. The signals are then studied to determine the current peaks and sidebands associated with fundamental motor current frequency. From these studies, one is able to monitor incipient rotor faults.
Other methods and apparatuses for monitoring electrical motors are disclosed by Saito et al. (U.S. Pat. No. 4,377,784), Bicknell et al. (U.S. Pat. No. 4,678,990), Schulz, Jr. et al. (U.S. Pat. No. 4,808,932), and Sekiguchi (U.S. Pat. No. 5,051,682).
While these methods can give secondary indications of rotor bar condition in some cases, they are not foolproof, and can be significantly influenced by other parameters. For instance, in the case of the raw current measurements the magnitude of the slip sidebands have been used. However, the slip sidebands are significantly smaller than the carrier (e.g., one-thousandth of the current load), and attempts to measure variations are inherently limited by the dynamic range of the measuring equipment. In the case of demodulated motor current, which certainly offers improved resolution of the slip sidebands, slip is often not detectable due to the presence of the large mechanical loads which confuse the picture.
In summary, several of the prior techniques rely upon secondary considerations rather than measuring the primary source itself. A need exists for an improved method for monitoring the condition of an operating electrical-motor-driven device.